Ultrasound imaging provides information about the interior of a subject. For example, ultrasound imaging can be used to generate an image of an internal anatomical structure (e.g., a blood vessel, etc.) and/or a flow of an internal structure (e.g., flow of blood in a vessel, etc.). B-mode with color flow mapping (CFM) is one approach for visually displaying anatomical structure with color indicia representing flow of structure (direction and magnitude) superimposed thereover. Other indicia (e.g., arrows, etc.) may additionally or alternatively be displayed to convey the direction and/or magnitude of the flow.
For CFM of blood flow, a pulse-echo field is transmitted and oscillates along the beam. Blood cells traversing the lumen of the vessel interact with the pulse-echo field and produce signals with frequency components that are proportional to the axial velocity of the blood flow. These signals are used to estimate relative blood flow, determined based on a phase shift between returning frequencies and transmitted frequency, with positive shifts indicating blood is moving away from the transducer, and negative shifts indicating blood is moving towards the transducer. The resulting data has been superimposed over a corresponding B-mode image.
The ability to visualize deep flow, flow of low amplitude, and very slow flow has been affected by transmit and post-processing parameters such as a color gain and a color flow processor wall (or high-pass) filter cut-off frequency. The working range of these parameters has been based on the extent to which the generated color flow images include color noise artifact (non-flow (e.g., noise) perceived as flow and presented in color as flow) and/or color flash artifact (non-flow motion above the wall filter cut off frequency in a displayed frame). Both parameters have been provided as manually adjustable parameters by a user.
Color gain affects an energy of the transmitted ultrasound signal. Generally, a higher color gain level leads to more of the true flow signal being detected, but a higher color gain level may also lead to a lower amplitude, non-flow signal, such as noise, being falsely perceived as flow and presented in color as flow, e.g., color outside of the vessel. Color noise has been mitigated by having the user turn down the color gain level through trial and error until an acceptable level of color noise in the displayed image is reached. Unfortunately, decreasing the color gain level may also result in lost true signal and thus undetected blood flow.
The wall filter cut-off frequency is set to separate high-amplitude, low-frequency stationary signal of moving tissue (e.g., a vessel wall, the chest due to breathing, the heart beating, the subject moving his/her arm, etc.) or slight transducer motion from low-amplitude, higher-frequency signal of moving blood-cells. If set to high, then low flow signal is filtered, and true signal is lost. If set too low, then the moving tissue signal may create large areas of color in one frame, which is absent from other frames such as a next displayed frame (hence, a color flash). Unfortunately, color flashes can be visually annoying to the user and/or may even mask or obscure the true signal.
In view of at least the above, there is an unresolved need for other approaches for visualizing color flow images.